Two populations or rats (Rattus norvegicus) were allowed to increase to three times optimum density. Painting one side of the habitat a darker color sufficed to divide the population into two subgroups, of which rats living on the dark painted side became dominant. Rats spent 90% of their time on their respective sides, thus enhancing (intra) group stability, and reducing (inter) group conflict. A 7-generation, 5-year study of a mouse (Mus musculus) population was also completed. When the population reached 8 times optimum density in the 6th generation (G6), 75% of G7 weaned came from 4 contiguous cells of the circular 8-cell habitat. Behavioral sink crowding together of mice in the other 4 cells continued through the next 8-month breeding period, with accentuated aggregation of mice in the nest boxes of all cells sufficient to preclude survival of any G8.